Factors influencing the rate of reactions
Every chemical reaction requires some time for its completion, but some reactions are very fast and some are very slow. Reactions between ions in solution without change in oxidation state are usually extremely fast. An example is the neutralization of an acid by a base, which proceeds as fast as the solutions can be mixed. Presumably nearly every time a hydronium ion collides with a hydroxide ion reaction occurs, and the number of collisions is very great, so that there is little delay in the reaction. The formulation of a precipitate, such as that of silver chloride when a solution containing silver ion is mixed with a solution containing chloride ion, may require a few seconds, to permit the ions to diffuse together to form the crystalline grains of the precipitate:
Ag+(aq)+Cl-(aq)→AgCl(c).
On the other hand, ionic oxidation-reduction reactions are sometimes very slow. An example is the oxidation of a stannous ion by a ferric ion:
2Fe3+ + Sn2+ →2Fe2+ + Sn4+
This reaction does not occur every time a stannous ion collides with one or two ferric ions. In order for a reaction to take place, the collision must be of such a nature that electrons can be transferred from one ion to another, and collisions that permit this electron transfer to occur may be rare.
Answer the following questions:
1.What reactions proceed fast?
2.What reactions are usually slow?
3.In what way is silver chloride formed?
Factors affecting the boiling point
The temperature at which a liquid boils, is dependent not only upon the nature of the liquid itself, but also upon the pressure prevailing at the time the boiling point is determined. An increase in the atmospheric pressure causes a rise in the boiling point of a liquid; a decrease results in a lowering of the bowling point.
Each individual substance capable of existing in the liquid state has its own definite boiling point. Consequently, this property is used for identifying or characterizing a substance in the same way that density and other physical properties are employed.
Inasmuch as the boiling point of a substance is affected by changes in pressure, it has been found necessary to set up a standard pressure at which all boiling points can be compared. A pressure of 760 mm Hg has been selected as the standard. Thus, if the boiling point of water is given as 100°C, the measurement is understood to have been made at a pressure of 760 mm Hg. Nevertheless, it is quite true to say that the boiling temperature of water is 70°C at a pressure of 233.8 mm Hg; it is also correct to say that carbon tetrachloride boils at a temperature of 70°C, but in this case the pressure is 620 mm Hg.
Answer the following questions:
1.What factors is the boiling point of a liquid dependent on?
2.How does the atmospheric pressure affect the boiling point?
3.What was the need of setting up a standard pressure?
Celsius versus Centigrade
The Ninth General Conference on Weights and Measures, held in October, 1948, adopted the name “Celsius” for the scale of temperature which had more commonly been called “centigrade”. This action, which had not been proposed in advance of the Conference, arose from a question regarding preferred usage in French, the sole official language of the Conference. The decision might be considered as applying strictly only to that language. In the interest of eventual uniformity of practice the use of “Celsius” appeared desirable, but it was not practicable to impose this term on those who preferred “centigrade”.
In preparation for the General Conference the National Bureau of Standard submitted a revised text defining the International Temperature Scale to supersede that adopted in 1927. The proposed text was drafted in English and, in accordance with common English practice as well as the official French text adopted in 1927, it used the name “centigrade”. This name was carried over into the French translation prepared for consideration by the Advisory Committee on Thermometry in May, 1948. However, in the printed report of that meeting, the term “centigrade” had, in most cases, been changed to ”centesimal”, the term that was used in the French law governing weights and measurements. When it was asked to choose between the two, the International Committee on Weights and the General Conference voted to substitute “Celsius”.
Answer the following questions:
1.What problems were discussed at the conference in 1948?
2.What name was adopted for the temperature scale devised by Celsius?
3.What was the reason for choosing a single name for the scale?
Bases
The subject of acids and bases has long been one of the most controversial in chemistry, and led to the development of an interesting series of theories. In the 17th century, during the infancy of experimental chemistry, acids and bases were defined or described on the basis of their behaviour. Thus, bases were substances that neutralized acids, turned plant dyes blue, had a bitter taste, and had a smooth or slippery feeling to the skin.
In the 18th century, following the discovery of oxygen by Joseph Priestley, Lavoisier advanced the idea that oxygen was the acidifying principle of all acids. Thereafter, the experimental approach was largely abandoned and emphasis was placed on the composition of substances instead of the phenomenological properties. The development of the hydrogen theory of acidity and Faraday’s studies of electrolytic conductance in the early 19th century led logically to the water-ion theory proposed by Arrhenius. By this concept a base may be defined as any hydroxyl compound which gives hydroxyl ions in water solution. Neutralization then involves the combination of hydroxyl ions with hydrogen formed by the acid, producing water and incidentally a salt. The role of solvent as an ionizing medium for acid-base reactions was emphasized. Although the theory under consideration was very useful and adequate for many reactions in aqueous solution, its many limitations soon became apparent.
Answer the following questions:
1.What substances were regarded as bases in the 17th century?
2.What important discoveries led to the water-ion theory?
3.What is a base according to Arrhenius?
Liquids and solutions
Solids and gases represent the extreme states of behavior of molecules. The liquid state can be thought of as an intermediate condition in which some of the properties found in either solids or gases are displayed. Liquids, like gases are isotropic and flow readily under applied stress, but like solids, they are dense, relatively incompressible, and have properties that are largely determined by the nature and strength of intermolecular forces. With respect to molecular order, liquids are substances considered to be intermediate between solids and gases. The fact that liquids are isotropic tells us immediately that they do not have the extended lattice structure and long-range order of solids. Yet, the density of a liquid is generally only 10% less than that of its solid phase; this must mean that the molecules in a liquid are packed together with some regularity, and do not exhibit the complete chaos associated with molecules in the gas phase.
One of the most important properties of liquids is their ability of acting as solvents. In the first place, liquid solutions provide an extremely convenient means of bringing together carefully measured amounts of reagents and of allowing them to react in a controlled manner. Second, the nature of the reactions which proceed and the speed at which they occur can be greatly influenced by the properties of the liquid solvent medium.
Answer the following questions:
1.What are the main states of matter?
2.What factors determine the properties of liquids?
3.Is there any difference in the density of solids, liquids and gases?